How to Efficiently Retrieve the Last Lines of a Text File in Java?

Efficiently Retrieving the Last Lines of a Text File in Java

Reading the last line of a large text file can be a challenging task, especially when attempting to avoid loading the entire file into memory. This article explores two efficient methods that accomplish this task without requiring traversal through the entire file.

Method 1: Tailing the File

The first method, known as "tailing," involves positioning at the end of the file and iterating backward character by character, recording the characters until a line break is encountered. This approach is efficient as it only requires processing a portion of the file. The following Java function implements this method:

public String tail(File file) {
  try (RandomAccessFile fileHandler = new RandomAccessFile(file, "r")) {
    long fileLength = fileHandler.length() - 1;
    StringBuilder sb = new StringBuilder();

    for (long filePointer = fileLength; filePointer != -1; filePointer--) {
      fileHandler.seek(filePointer);
      int readByte = fileHandler.readByte();

      if (readByte == 0xA) { // line break (LF)
        if (filePointer == fileLength) {
          continue;
        }
        break;

      } else if (readByte == 0xD) { // carriage return (CR)
        if (filePointer == fileLength - 1) {
          continue;
        }
        break;
      }

      sb.append((char) readByte);
    }

    String lastLine = sb.reverse().toString();
    return lastLine;
  } catch (Exception e) {
    e.printStackTrace();
    return null;
  }
}

Method 2: Tailing N Lines

The second method extends the first by returning the last N lines of the file. It operates similarly to the tailing method but counts line breaks to determine the desired number of lines to retrieve. The following Java function implements this method:

public String tail2(File file, int lines) {
  try (RandomAccessFile fileHandler = new RandomAccessFile(file, "r")) {
    long fileLength = fileHandler.length() - 1;
    StringBuilder sb = new StringBuilder();
    int lineCount = 0;

    for (long filePointer = fileLength; filePointer != -1; filePointer--) {
      fileHandler.seek(filePointer);
      int readByte = fileHandler.readByte();

      if (readByte == 0xA) { // line break (LF)
        if (filePointer < fileLength) {
          lineCount++;
        }
      } else if (readByte == 0xD) { // carriage return (CR)
        if (filePointer < fileLength - 1) {
          lineCount++;
        }
      }

      if (lineCount >= lines) {
        break;
      }

      sb.append((char) readByte);
    }

    String lastLines = sb.reverse().toString();
    return lastLines;
  } catch (Exception e) {
    e.printStackTrace();
    return null;
  }
}

Usage

These methods can be invoked to retrieve the last line or the last N lines of a large text file. For example, consider the following usage:

File file = new File("D:\stuff\huge.log");
System.out.println(tail(file));
System.out.println(tail2(file, 10));

Caution

Note that this approach may not handle all unicode characters correctly due to the reversal of compound characters upon reversal. Refer to the linked article for more information about this potential issue.

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